Macrophages (MPhi) and endothelial cells in atherosclerotic lesions express two enzymes arising from the acid sphingomyelinase (SMase) gene: secretory (S) and lysosomal (L-) SMase. S-SMase promotes lipoprotein aggregation, which leads to subendothelial lipoprotein retention and MPhi foam cell formation. New data,however, suggest that lysosomal (L-) SMase facilitates cholesterol efflux. Thus, we hypothesize that while S-SMase promotes foam cell lesions, L-SMase plays a role in foam cell regression. Moreover, acid Smase deficient MPhis are protected from free cholesterol-induced MPhi apoptosis, suggesting a role for S- or L-SMase in lesional MPhi death. Our overall objective is to explore these hypotheses and other roles of S- and L-SMase in atherogenesis using genetically engineered mouse models and cultured MPhis. In Aim 1, acid SMase knockout mice will be used to define further the net effect of S- and L-SMase deficiency on foam cell lesions and on aortic LDL retention, degradation, and aggregation in vivo. In addition, we will create two new mouse models specifically to define the roles of S- vs. L-SMase in atherogenesis. In Aim II, we will address three areas in which S- and L-SMase may influence Mo biology related to atherogenesis: (1) cDNA microarrays will be used to test the hypothesis that MPhi interaction with matrix-retained and S-SMase-aggregated LDL alters gene expression; (2) the mechanisms and consequences of SMase-mediated death will be investigated in cultured MPhis and in vivo; and (3) in collaboration with Project 6, we will explore the mechanisms of defective cholesterol efflux in SMase-deficient MPhis. In Aim 1, through a large multiethnic study, we will explore our finding that high plasma sphingomyelin is a potent and independent risk factor for coronary artery disease in humans. To investigate a possible mechanism of this association, we will test the hypothesis that human remnant lipoproteins are relatively rich in sphingomyelin, rendering them susceptible to S-SMase and converting them into potent inducers of foam cells following S-SMase treatment. This work is intimately related to the overall theme of the SCOR-the investigation of arterial-wall molecules involved in atherogenesis using genetically manipulated mouse and cell-culture models-and will involve collaborations with Projects 6, 7, and 8 and use of the Gene Expression, Clinical, and Pathology Cores.